1. Field of the Invention
The present invention relates to a single crystal heat treatment method; and more specifically, it relates to a heat treatment method for a single crystal used in a single crystal scintillation detector (scintillator) for detecting radiation such as gamma radiation and the like in fields such as radiology, physics, physiology, chemistry, mineralogy, and oil exploration, including medical diagnostic positron CT (PET) scanning, observation of cosmic rays, and exploration for underground resources.
2. Related Background of the Invention
Because scintillators that use cerium-doped gadolinium orthosilicate compounds have a short fluorescent decay time and also a large radiation absorption coefficient, they have found applications as radiation detectors for positron CT and the like. The light output of these scintillators is greater than that of BGO scintillators, but only about 20% of the light output of NaI (T1) scintillators, and further improvement is needed in that area.
Recently, scintillators using single crystals of cerium-doped lutetium orthosilicate, which are generally represented by the formula LU2(1−x)Ce2xSiO5, have been disclosed (Japanese Patent Registration No. 2852944, and U.S. Pat. No. 4,958,080). In addition, scintillators using single crystals of the compound represented by the general formula Gd2−(x+y)LnxCeySiO5 (wherein Ln represents Lu or a species of rare earth element) have been disclosed (see Japanese Patent Application Laid-open No. 7-78215 and U.S. Pat. No. 5,264,154). It is known that in these scintillators not only is the crystal density increased, but also the light output of cerium-doped orthosilicate single compounds crystals is increased, and the fluorescent decay time can be shortened.
Furthermore, Japanese Patent Registration No. 2701577 discloses a heat treatment method that increases scintillator properties such as light output and energy resolution capability and the like of single crystals of cerium-doped gadolinium orthosilicate compounds. This heat treatment method is one wherein a heat treatment is performed in an oxygen-poor atmosphere at a high temperature (a temperature ranging from 50° C. to 550° C. lower than the melting point of the single crystal). According to this document, scintillation properties are increased by an effect wherein tetravalent cerium ions, which are an impediment to scintillation luminescence, are reduced to a trivalent state.